The present invention relates to optical filters, and more particularly, to tunable Fabry-Perot optical resonators, filters and lasers constructed therefrom.
Tunable optical resonators are utilized in optical communication systems and in the construction of lasers. Optical filters and lasers based on Fabry-Perot resonators can be constructed using microelectromechanical machining (MEM) techniques, and hence, can, in principle, provide an economically attractive tunable filter or tunable laser. In such devices, a Fabry-Perot resonator cavity is formed between two mirrors. One of these mirrors is flat and located on a semiconductor substrate. The other mirror may be curved and is suspended on a number of micro-mechanical cantilevers. Application of a tuning voltage between the cantilevers and the substrate causes the suspended mirror to move towards the fixed mirror on the substrate, thereby reducing the spacing between the two mirrors of the Fabry-Perot resonator. Since the filter""s bandpass frequency is determined by the mirror spacing, a reduction in spacing between the two mirrors causes the resonant optical frequency of the cavity to increase. The shift in the resonant frequency enables the device to be used directly as a tunable bandpass filter. If an optically-pumped or electrically-pumped optical gain medium (active region) is placed in the cavity, the device becomes a tunable laser, with the lasing wavelength controlled by the resonant frequency of the Fabry-Perot cavity.
Prior art MEM Fabry-Perot filters exhibit a significant amount of noise, which limits the usefulness of these devices. The noise results from mechanical vibrations in the mirror connected to the cantilevers. This noise causes variations in the spacing between the mirrors, which in turn, causes the resonant frequency and amplitude of the light emitted from the filter to exhibit a corresponding noise spectrum. The noise broadening of the resonant frequency can double the width of the passband response of a filter and can substantially increase the linewidth of a laser constructed from such a filter, thereby making the filter or laser unsuitable for many applications.
Broadly, it is the object of the present invention to provide an improved MEM Fabry-Perot resonator.
It is a further object of the present invention to provide a MEM Fabry-Perot resonator that has reduced noise.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a tunable optical cavity constructed from a fixed mirror and a movable mirror. The fixed mirror is attached to a substrate having a first electrically conducting surface. A support member having the moveable mirror supported thereon and having a second electrically conducting surface, is suspended above the substrate. A circuit applies an electrical potential between the first and second electrically conducting surfaces thereby adjusting the distance between the fixed and movable mirrors. The fixed mirror and the moveable mirror are positioned such that the mirrors form the opposite ends of the optical cavity. The distance between the fixed mirror and the moveable mirror is a function of the applied electrical potential. The support member has physical dimensions that are chosen such that the amplitude of thermally induced vibrations in the support member are less than 0.01 percent of the wavelength of the resonating light. The support member is preferably a thin film membrane having the moveable mirror attached thereto and having openings communicating between the top and bottom surfaces of the membrane. The openings are located so as to reduce the vibration amplitude of the moveable mirror at any mechanical resonances of the membrane. The openings occupy less than 90% of the surface area of the membrane.